Can a TPS ona 91 xploder be so bad that it can stop the ignition from firing? if so can it be explained? Thanx.

 

No. Jerry suggested the crank sensor right? TPS has nothing to do with ignition (directly), only A/F mixture. What makes you think that? Have you gone through the ignition circuit completely at this point? Crank sensor through the coil/wire/plugs? You don't even need the PCM to get a spark. The crank sensor, ICM and coil all have enough info to get a basic spark.

Now, load is affected by TPS. Load changes the spark curve, the SPOUT signal from the PCM to the ICM that fine tunes the spark curve/advance. Good question but I don't think it can possible stop a spark all together - might screw up the timing and get the car to run like crap - but also would have worse affect on the A/F mix.

Maybe the TPS is not allowing any fuel?

 

I know, it doesn't make sense but a sharp rap on the the TPS whilst running killed the spark, it is still dead.

 

That's a different story. Maybe the system check with Key On disables spark if the TPS doesn't respond with expected feedback. Sounds extensive but they did write some complicated program. Have you checked for codes again since this event?

 

It gave an 18 - spout circuit open. Which I verified as i get a nice .5 vac input to edis from vrs but no tach sig out. All wires from and to EDIS rang out fine. Think i will disconnect battery and toss in a new tps. 3 weeks ago a bad bap sensor killed the spark. It shares a common source with TPS from the pcm. Very frustrating this one.

 

There is a SPOUT shorting bar near the ICM - is that still there? I don't know why Ford needed this, maybe to troubleshoot without the PCM interferring. I got a SPOUT open when I had the SCT chip in - from KOEO test. SCT was clueless about this by the way - morons. But of course my shorting bar was still there. What year/motor truck is this again? I have a short memory. I'll check out the wiring diagram tomorow. Must be a 91 or 92 with a BAP in it.

 

My PCM pinnout diagram isn't matching up to the autozone wiring diagram. I see what you mean that the BAP and TPS share the same reference voltage pin. Still don't see the correlation to spark. Would be intersting to see if there are any codes and KOEO/KOER test results. I wonder what a new PCM would do.

 

Giving out 2 digit codes and with a BAP, it almost has to be a '91 (my '92 gives off three digit codes and doesn't have a BAP). I also took and unplugged the TPS on my '92. Still started and ran ok (idle was a little high and the CEL came on). I really don't think a bad TPS would prevent you from starting the engine. (For that matter, a fault in the BAP shouldn't prevent it from starting, either).

Another observation: we have a fault that appears to be jumping from component to component (BAP to TPS to ???). This suggests to me that the components themselves aren't at fault, but that something in the wiring or elsewhere is the real cause.

Bad grounds do funny things (once I unbolted and/or unplugged the fuel pump ground to clean it and the wipers went crazy on me until I put it back together). Are you absolutely certain the ICM, PCM, engine block, and ignition coil are all properly grounded?

My code lists suggest that a CM 18 is an erratic TACH signal rather than SPOUT (though I expect they are closely related). TACH signal is generated by the crankshaft position sensor (CKP). Are you certain the wiring between the ICM and the CKP is good? Is the CKP good (while not real common, I have heard of them failing)?

Those are a few of my thoughts. It will be interesting to see what you determine is at fault.

 

Yeah it's a 91. Said so in first post. I was too stupuid to look.

Savage - you said you get .5 VAC, sure it's not DC voltage? Make sure your Fluke meter is on DC not AC.

 

AC voltage, unless I missed one, all grounds are squeaky clean. Spout shunt is in place it is there to provide a base timing check. Engine can run with or without it. I'm stumped. Have a new from Ford TPS, battery is disconnected, after din dins will go out and try it, not confident though. Thanx for the input so far.

 

Tried the new TPS no go. For the record, all other sensors have been swapped with known good ones even the ICM and coil pack. What is really frustrating is you can see a faint spark jump thru the neon bulb inline spark tester as it is cranking, so signals are getting thru just not enough power. Methinks this is one for Sherlock Holmes. By the way, did these things have a factory security device in 91? if so, how would I recognize it?

 

I don't think so Sherlock. In 1993 when I bought mine, the dealer offered an alarm but it was aftermarket - no Ford options. So I doubt they had one in 1991.

Like Mrshorty said, you checked all grounds right? Does sound like a grounding issue. Next step would be a new PCM. I bought a spare on eBay for $40 a while back just in case I ever run into something like this.

Also - there's an octane shorting bar which I believe will give you 10 degrees base timing when removed. I'm not sure how SPOUT jumper plays but sure, if you remove it, the PCM then has no influence on the spark curve.

 

All other sensors? That's a lot of money spent on guessing. Looks like it's time for some real diagnosis. I propose one of 2 options:

1) Pay someone to diagnose this for you. Of course, you have to get someone good. You don't want someone else who's just guessing as well.
2) Go to the library with a pocketful of loose change. Many libraries, in the reference section, will carry Motor or Mitchell manuals. Locate the appropriate "engine performance and troubleshooting" (or equivalent) manual. Locate the circuit test for CM18 on a '91 4.0 engine. Read it, study it, photocopy it, then take it home and see what you can come up with.

FWIW, I located my gtest.pdf file for a '95 Explorer. Among the first tests they suggest for a no start is to see if the TPS is seeing Vref. If the computer isn't generating Vref, then they have you figure out why the computer isn't outputting Vref (Vref is also used by the BARO sensor). Are you getting Vref out of your computer? Another thought, if you clear the CM 18 and try to start the engine, does the CM 18 come back immediately or not?

 

I was kind of wondering about that reference voltage to. It sound be 5 V DC not AC. So that AC reference is a bit confusing. Looking like PCM to me.

 

Ten four I've tried it with both shorting bars in and out one and both, no go. All grounds polished. I've tried a PCM from a 91 aerostar 4.0 not exact match but no go either. The thing that is getting me is the weak spark thru the tester. The coils are firing albeit faintly my other coil pack makes no diff.